When information is missing or inconsistent, the Census Bureau uses a method called imputation to assign values. Imputation relies on the statistical principle of "homogeneity," or the tendency of households within a small geographic area to be similar in most characteristics. For example, the value of "rented" is likely to be imputed for a housing unit not reporting on owner/renter status in a neighborhood with multiunits or apartments where other respondents reported "rented" on the census questionnaire. In past censuses, when the occupancy status or the number of residents was not known for a housing unit, this information was imputed.

An operation which allows respondents to use the Census Bureau's Internet site to (1) ask questions and receive answers about the census form, job opportunities, or the purpose of the census and (2) provide responses to the short form.

Interpolation frequently is used in calculating medians or quartiles based on interval data and in approximating standard errors from tables. Linear interpolation is used to estimate values of a function between two known values. Pareto interpolation is an alternative to linear interpolation. In Pareto interpolation, the median is derived by interpolating between the logarithms of the upper and lower income limits of the median category. It is used by the Census Bureau in calculating median income within intervals wider than $2,500.

The data on work status in 1999 were derived from answers to long-form questionnaire Item 30a, which was asked of a sample of the population 15 years old and over. People 16 years old and over who worked 1 or more weeks according to the criteria described below are classified as "Worked in 1999." All other people 16 years old and over are classified as "Did not work in 1999." Some earnings tabulations showing work status in 1999 include 15 year olds; these people, by definition, are classified as "Did not work in 1999."

The data on weeks worked in 1999 were derived from answers to long-form questionnaire Item 30b, which was asked of people 15 years old and over who indicated in long-form questionnaire Item 30a that they worked in 1999. The data were tabulated for people 16 years old and over and pertain to the number of weeks during 1999 in which a person did any work for pay or profit (or took paid vacation or paid sick leave) or worked without pay on a family farm or in a family business. Weeks on active duty in the armed forces also are included as weeks worked.

Median weeks worked in 1999 divides the weeks worked distribution into two equal parts: one-half of the cases falling below the median weeks worked and one-half above the median. Median weeks worked in 1999 is computed on the basis of a standard distribution (see the "Standard Distributions" section under "Derived Measures"). Median weeks worked is rounded to the nearest whole number. (For more information on medians, see "Derived Measures".)

The data on usual hours worked in 1999 were derived from answers to long-form questionnaire Item 30c. This question was asked of people 15 years old and over who indicated that they worked in 1999 in Question 30a, and the data are tabulated for people 16 years old and over. The respondent was asked to report the number of hours usually worked during the weeks worked in 1999. If their hours varied considerably from week to week during 1999, the respondent was asked to report an approximate average of the hours worked each week. People 16 years old and over who reported that they usually worked 35 or more hours each week are classified as "Usually worked full time"; people who reported that they usually worked 1 to 34 hours each week are classified as "Usually worked part time."

Median usual hours worked per week in 1999 divides the usual hours worked distribution into two equal parts: one-half of the cases falling below the median usual hours worked and one-half above the median. Median usual hours worked per week in 1999 is computed on the basis of a standard distribution (see the "Standard Distributions" section under "Derived Measures"). Median usual hours worked per week is rounded to the nearest whole hour. (For more information on medians, see "Derived Measures".)

The aggregate usual hours worked per week in 1999 is the number obtained by summing across the usual hours worked values of all people who worked in 1999. (Note that there is one usual hours value for each worker, so the number of items summed equals the number of workers.)

Mean usual hours worked per week is calculated by dividing the aggregate number of usual hours worked per week worked in 1999 by the total number of people who worked in 1999. Mean usual hours worked per week is rounded to the nearest tenth. (For more information on means, see "Derived Measures".)

'Full-time, year-round workers' consists of people 16 years old and over who usually worked 35 hours or more per week for 50 to 52 weeks in 1999. The term "worker" in these concepts refers to people classified as Worked in 1999 as defined above. The term worked in these concepts means "worked one or more weeks in 1999" as defined above under "Weeks Worked in 1999."

It is probable that data on the number of people who worked in 1999 and on the number of weeks worked are understated since there was probably a tendency for respondents to forget intermittent or short periods of employment or to exclude weeks worked without pay. There may also have been a tendency for people not to include weeks of paid vacation among their weeks worked, which would result in an underestimate of the number of people who worked "50 to 52 weeks."

The data on weeks worked collected in Census 2000 are comparable with data from the 1960 to 1990 censuses, but may not be entirely comparable with data from the 1940 and 1950 censuses. Starting with the 1960 census, two separate questions have been used to obtain this information. The first identifies people with any work experience during the year and, thus, indicates those people for whom the question about number of weeks worked applies. In 1940 and 1950, the questionnaires contained only a single question on number of weeks worked. In 1970, people responded to the question on weeks worked by indicating one of six weeks-worked intervals. In 1980 and 1990, people were asked to enter the specific number of weeks they worked.

The terms "worker" and "work" appear in connection with several subjects: employment status, journey-to-work, class of worker, and work status in 1999. Their meaning varies and, therefore, should be determined by referring to the definition of the subject in which they appear. When used in the concepts "Workers in Family," "Workers in Family in 1999," and "Full-Time, Year-Round Workers," the term "worker" relates to the meaning of work defined for the "Work Status in 1999" subject.